The present invention relates generally to infrared (IR) cameras and detectors. More particularly, the present invention relates to lightweight infrared (IR) cameras and detectors.
Infrared cameras and detectors in general, and microbolometer cameras in particular, are well known to those skilled in the art. See, for example, U.S. Pat. Nos. 5,688,699; 5,999,211; 5,420,419; and 6,026,337, all of which are incorporated herein by reference. Infrared microbolometer cameras typically include an array of infrared sensitive sensing detectors, each having a resistance that changes with temperature, with each detector having an infrared absorber that may be formed in several ways. See, for example, U.S. Pat. Nos. 5,939,971 and 5,729,019, herein incorporated by reference.
During operation, the incoming infrared radiation heats each sensing detector in proportion to the amount of infrared radiation received. The sensing detectors are then queried, typically one by one, to determine the resistance of the sensing detectors, and thus the amount of infrared radiation received. Typically, supporting electronics are provided with the camera to process the detector output signals, provide calibration and compensation, and provide a resulting image.
Because heat is used to measure the amount of incoming infrared energy, changes in the ambient temperature of the microbolometer array can significantly affect the detector signals. To compensate for this, many infrared cameras or detectors have a thermoelectric stabilizer to regulate the temperature of the array. In one example, thermoelectric stabilizers are used to maintain the array temperature at a known value. A limitation of using thermoelectric stabilizers is that they can draw significant power and can add significant weight to the system.
Because of manufacturing tolerances, each sensing detector in the camera may have a slightly different zero point than other detectors within the system. To compensate for these detector-to-detector differences, many infrared cameras or detectors have a means for providing a zero radiation baseline value, which is made available to interpret or calibrate the detector output signals. One method for providing the zero radiation baseline is to use a shutter or chopper to periodically block the incoming infrared energy. When the shutter or chopper is activated, a zero radiation baseline is read and stored. A limitation of this approach is that the shutter or chopper can add significant complexity and weight to the system, which for some applications, can be particularly problematic. Another approach for providing a zero radiation baseline is to periodically point the camera at a uniform infrared source such as the sky. This, however, can require significant control circuitry to periodically change the direction of the camera, again adding weight to the system.
For some applications, the weight of the infrared camera can be important. For example, in lightweight micro air vehicle (MAV) applications, the weight of the infrared camera can significantly impact the size, range and other critical performance parameters of the vehicle. For these and other applications, a lightweight infrared camera would be highly desirable.
The present invention overcomes many disadvantages of the prior art by providing a lightweight infrared camera. This is accomplished primarily by eliminating the shutter or chopper, eliminating the thermoelectric stabilizer, using lightweight materials and lightweight packaging techniques, and/or moving some of the calibration, compensation, processing and display hardware from the camera to a remote station.
In one illustrative embodiment of the present invention, the infrared camera includes a microbolometer array as the radiation sensing device. The microbolometer array includes a plurality of addressable radiation sensing detectors each having an output that depends on the intensity of the infrared radiation that strikes the detector.
To reduce the weight of the infrared camera, the measured infrared signals may be transmitted to a remote station in analog or digital form. The signals may be transmitted by wireless or optical fibers or wires as best suits the application. The remote station receives the transmitted signals, and formats the signals into an array that corresponds to the original microbolometer detector array. The remote station may include the necessary processing hardware for compensating both inter-detector differences and variations in the ambient temperature of the transmitting detector array. A temperature sensor also may be provided near the microbolometer array, which can send a temperature signal that is transmitted to the remote station for use in signal calibration and compensation, if desired. By moving the calibrations compensation and/or processing hardware from the infrared camera to the remote station, significant weight savings can be realized in the infrared camera.
The calibrating and compensating values are typically dependent upon the temperature of the microbolometer array, typically being different for each bolometer array temperature and for each individual sensing detector on the array. The remote station may select the proper calibrating and compensating values to apply. The required numbers may be stored in the remote station, or generated for each individual sensing detector using algorithms which use the array temperature as a variable.
In camera applications in which the scene has known statistical properties, for example, when each sensing detector views a target which on average is identical to every other sensing detector, as is usually the case in MAV and many other moving-vehicle applications, the compensating and calibrating values may also be computed within the ground station by using multiple measured values of each sensing detector signal.
Another way to reduce the weight of the infrared camera is to provide the microbolometer in an integrated vacuum package (IVP). An integrated vacuum package may include an infrared transmitting cover that includes a cavity that fits over the microbolometer detector array. Silicon is a typical cover material. The silicon cover is bonded to the microbolometer substrate to collectively form a lightweight vacuum package. In a preferred embodiment, the silicon cover does not extend over the bonding pads of the microbolometer. Configured in this way, the IVP may be directly bonded to a motherboard, with wire bonds, bump bonds or other bonding mechanisms used to directly connecting the bonding pads of the microbolometer to bond pads on the motherboard. Motherboards are typically ceramic. This is known as xe2x80x9chybridizingxe2x80x9d the IVP with the motherboard. This may eliminate the need for a conventional chip carrier, which may further reduce the weight of the camera.
It is also contemplated that any supporting electronics in the camera, such as A/D converters and/or transmitting circuitry, may be hybridized with the ceramic motherboard. That is, rather than including the supporting electronics in conventional packages, the integrated circuit dice of the supporting electronics may be directly bonded to the ceramic motherboard, with wire bonds, bump bonds or the like connecting the supporting electronics to the motherboard. This may also reduce the weight of the camera.
The infrared camera may also use a lens system. The lens system is used to focus the incoming infrared radiation on the microbolometer array of detectors. The lens is typically a germanium lens, and may be a singlet, doublet, or triplet. The lens is preferably spaced from the ceramic motherboard by lightweight supports made from a material such as titanium. The use of doublets or singlets can further reduce the weight of the infrared camera. If doublets or singlets are used, the resulting image blur may be removed by the ground station.